Lambda probes of the Nernst type have been known since the start of catalyzer technology with lambda control in motor vehicles and will be used most often also in the next few years. These lambda probes have a very steep characteristic; that is, the probe voltage changes only slightly with increasing rich mixture in the case of the measurement of a rich mixture and changes only slightly with lean mixtures with increasing lean mixture. However, in contrast, in the transition from a rich to a lean mixture within a very tight mixture range, a change of several 100 mV occurs. Typically, the probe voltage is at approximately 850 mV when measuring a rich mixture and at approximately 100 mV when measuring a lean mixture. The actual measured voltages fluctuate considerably however from one probe to another. A first probe can display 1 V when measuring a rich mixture and another probe can measure up to approximately -80 mV when measuring a lean mixture.
The above-mentioned characteristics specific to probes are quite insignificant for the control since with these probes typically only the inquiry is made as to whether the probe voltage lies above or below 450 mV. The characteristics specific to the probe lead however to problems when checking the connecting circuit as to faults especially if a short circuit to ground is present. 0 V is measured for short circuits to ground. This however is also a plausible measurement value since, as mentioned in the above paragraph, the voltage measured during lean can be 0 V or even lie therebelow with negative voltages not being evaluated by the typical evaluation circuits; that is, the negative voltages also lead to a measured voltage of 0 V.
In order to reliably determine short circuits to ground, the conventional practice is that when the probe voltage has a value of 0 V over a longer time span, the fuel/air mixture is arbitrarily enriched. If the probe signal does not respond to this enrichment, this is a reliable indication that a ground short is present. It is disadvantageous in this test method that the mixture must be enriched which leads to an increased exhaust of toxic gas and also causes other disadvantages.
Lambda probes are mostly so mounted that they detect the gas composition in the exhaust gas flow forward of a catalyzer. However, it is known for example from U.S. Pat. No. 4,622,809 that, in addition, a lambda probe can be mounted rearwardly of the catalyzer and, with the aid of this probe, the conversion capacity of the catalyzer is monitored. As long as the catalyzer converts well, a gas mixture of very uniform composition flows past the rearward lambda probe. For an engine controlled to the lambda value one, a voltage of approximately 450 mV is measured notwithstanding the relatively steep characteristic of the probe, since the lambda value rearward of the catalyzer is quite constant at the value one. This fact leads to difficulties when checking the operability of the probe rearward of the catalyzer since, for a probe voltage which is continuously uniform, it is unclear whether the connecting circuit for the rearward probe is defective or if the catalyzer converts so well that there are no changes. A check is possible also in this case in that over a longer period of time, a mixture is generated deviating from the lambda value one. This leads to the disadvantages already mentioned above.